Supramolecular mesogens

Fig. 6 Structures of the non-mesogenic bis-pyridyl (2) and bis-benzoic acid (3) monomers studied by Griffin et al. [67] and the proposed structure of the main chain liquid crystalline supramolecular polymer (2-3) highlighting the length of supramolecular mesogen...

Fig. 4. Self-assembly process of benzoic acid 2 and bipyridine 4, and induction of a mesophase by the formation of a supramolecular mesogen...

As this supramolecular mesogen does not form a simple rod or disc shape, the molecular arrangements of calamitic and columnar mesophases can be tuned when appropriate substituents are introduced into either of the components [61, 63]. Triazines and carboxylic acids are complexed to form doubly H-bonded mesogens which display columnar mesophases, e.g. 22 [64, 65]. 

An initial approach to supramolecular H-bonded mesogenic polymer complexes involves a polyacrylate with 4-oxybenzoic acid moieties via a hexamethylene spacer 29 [26]. The 1 1 complexation of the side chain of the polymer and stilbazole 3 n - 2) (nematic, 168-216 °C) results in the formation of an extended supramolecular mesogen in the side chain (Fig. 11). Side-chain polymer complex 30 exhibits a nematic phase up to 252 °C, which shows that a significantly stabilized mesophase is achieved by the complexation of two different components. Liquid-crystalline properties have been examined for the series of complexes formed between polyacrylates and trans-4-alkoxy-4 -stilbazoles [33, 78]. Figure 12 shows transition temperatures against the carbon number of the alkyl chain for the series of complexes 31 [33]. They exhibit thermally stable smectic liquid-crystalline phases. For example, smectic E, B, and A phases are observed until 192 °C after the glass transition at 38 °C for the complex with m = 6 [78a]. 

Double and triple hydrogen bonds can also be used to design the supramolecular mesogens. Complexes 33 [49] and 34 [21] have shapes between rodlike and dis-cotic mesogens. Complex 33 exhibits monotropic calamitic or discotic phases. Triple hydrogen bonded complex 34 is a discotic mesogen with a columnar phase from 200 °C to room temperature on cooling. 

Figures Early examples of supramolecular mesogenic complexes 1 and 2 formed through complementary...

Figure 7 Supramolecular mesogenic complexes 8 and 9 using hydrogen bonding between phenol and pyridine.

Figure 9 Supramolecular mesogenic complex 11 using halogen/pyridine interactions.

Figure27 Cross-linked supramolecular mesogenic polymer 37.

Doubly hydrogen-bonded supramolecular mesogenic complexes have been derived by the formation of the hydrogen bonds between 2-aminopyridyl and carboxylic acid moieties [43,44]. Complex 24 sharply melts... 

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